This invention relates to raster distortion correction arrangements utilized with deflection yokes of the self-converging type.
U.S. Pat. No. 3,721,930 discloses a deflection yoke suitable for use with a three in-line beam color television picture tube for producing substantial convergence of the three beams at all points on the raster without the use of dynamic convergence apparatus. This is accomplished essentially by selecting the winding distribution of the coils for producing respective generally pincushion and barrel-shaped horizontal and vertical deflection fields which are so proportioned as to balance the convergence condition on the axis and in the corners of the raster. This teaching has been utilized in television receivers utilizing beam deflection angles of 90 and 110 degrees.
Pincushion distortion is associated with most television displays including the self-converging type described above. This distortion, generally, is caused by the picture tube geometry and the shape of the magnetic deflection fields. The geometrical distortion occurs because all points of the viewing screen are not equidistant from the deflection center of the yoke. This geometric pattern distortion of the raster increases with increasing beam deflection angle. The magnetic deflection fields influence pincushion distortion in the following manner: if the shape of the deflection field is pincushion the raster distortion is reduced; if the field is barrel-shaped the raster distortion is increased.
It is known that pincushion distortion can be substantially eliminated by using pincushion shaped deflection fields, it is also known that the shape of the deflection field near the front or beam exit is the most sensitive part of the field to control pincushion pattern distortion. This is because the beams are closer together at the front of the yoke than at the rear and the beams also are the most deflected at the front of the yoke. This follows from the third order beam theory that raster distortion is a product of the square of the amount of beam deflection from the central longitudinal axis of the yoke and the amount of transverse nonuniformity of the deflection field. This nonuniformity is generally expressed as the H.sub.2 function and is the third order right angle component of the deflection field.
Also, it is known that the deflection field astigmatism, which influences the beam convergence, is best controlled by altering the deflection field further back from the beam exit end of the yoke. Since astigmatism is a linear product of the amount of beam deflection and the H.sub.2 field it can actually be compensated for by altering the deflection field anywhere along the yoke central axis, but since the front portion of the yoke is designed for pattern correction the astigmatism correction must be accomplished elsewhere. Saddle-type vertical deflection coils lend themselves to being pincushion corrected because the coil winding cavity on the winding machine can be controlled relatively easily. However, many deflection yokes in use today utilize a toroidally wound vertical deflection winding. Toroidal coils are generally wound having radial conductor turns. Radial turns are those which lie in a plane containing the central longitudinal axis of the deflection yoke. To wind a nonradial toroidal deflection winding with presently available toroidal coil winding machines would require either a significant reduction in the winding speed or increased complexity of the machinery involved, or both.
Obviously, another problem encountered when attempting to wind either a saddle or toroidal pincushion corrected winding is the maintaining of the proper astigmatic field required for self-convergence of the beams.